I was a little surprised to find out that the IEEE recommends using "ac" and "dc" for alternating/direct current rather than "AC" and "DC." (Google "Recommended Unit Symbols, SI Prefixes, and Abbreviations - IEEE") Apparently non-capitalized is the "correct" way but I've mostly seen them capitalized. Capitalized seems to make more sense since they're acronyms... Does anyone know why they're not supposed to be capitalized? Also, my main question: are "ac" and "dc" really preferred in cases where I can choose either way? (I'm not submitting to IEEE) From my experience, most people seem to use the upper-case acronyms, and I suspect most people will be thrown off by the lower-case acronyms even if they're technically correct.